Proccplmlic l.dlx's of l']|)cii':i ( 'iiicrcii. 433 



In sections these elevations are seen to be lined with mesoderm, 

 and enclose a portion of the cffilomic cavity, a fact which indicates 

 their appendage-like character. This double origin of the rostrnm 

 was shown by Schimkewitsch, who also pointed out the fact that, 

 as indicated by their structure, these prominences re])resent a pair 

 of cephalic appendages. 



About this time numerous pits appear in the ganglionic areas 

 of the cephalic plate (?i. b.). Each pit arises as an independent 

 structure, and is surrounded by a well-marked circle of nuclei. Tn 

 surface views they appear to be like small, light cells, arranged with 

 considerable r(>gularity in the ectoderm. Viewed superficially they 

 appear to contain no nuclei ; but on close inspection nuclei may be 

 discerned lying below the surface of the depression, thus indicating 

 the position of the cells of which it is composed. 



In sections these pits are seen to be surrounded by cells having 

 a considerable regularity of arrangement. The hollow of the pit 

 is filled with a clear substance, a])parently secreted from the sur- 

 rounding cells. 



At first sight these ])its l)car a chisc I'cscniblaiicc 1<» lli(^ structures 

 wdiich Wheeler found in Xi pliidhnn . lo which he api)licd the name 

 of "neuroblasts." The neuroblasts of Wheeler are described as 

 being "numerous, large, light cells in the surface of the ectod(>rm" 

 from which the elements entering into the strnctun; of the sense 

 organs are derived by pn.lifcralion. In Eprh-a, however, the pit^ 

 are, as has been indicated,, more or less regularly arranged depres- 

 sions of the ectoderm; the cells snrrounding each depression being 

 grouped together in a manner which strongly suggests the way the 

 optic cells are arranged in a simple ocellus. 



Patten has ex])i-essed the belief that these ])its represent primi- 

 tive sense organs, rather than being, like the neuroblasts of Wheeler, 

 the points of origin for numerous sensory elements. Patten has 

 shown that these primitive sense organs form at a later period, by 

 proliferation and transformation, special groups of ganglion cells. 

 Moreover, since, as the same autlior has clearly shown, the minute 

 structure of these pits is tlie same as that of true ])erii)beral sense 

 organs which occnr at the base of the legs in scor])ions, one can 

 scarcely avoid adopting his point of view. 



